1. Field of the Invention
The present invention relates to a retrofocus type bright and wide-angle lens system which has a photographing field angle of at least 75.degree. and an F number on the order of 2.
2. Description of the Prior Art
Along with the recent progresses made in lens design, shaping and manufacturing, zoom lens systems are being adopted even for most single-lens reflex cameras which are expected to form images of high qualities. However, it is difficult to correct aberrations sufficiently favorably within vari-focal ranges of bright zoom lens systems which have small F numbers and it cannot be said that aberrations are corrected in such zoom lens systems so sufficiently as those in lens systems having fixed focal lengths. Therefore, attempts are being made to configure lens systems having fixed focal lengths so as to have large apertures and form images of higher qualities than those available with zoom lens systems.
High optical performance is desired, in particular, for lens systems which have photographing field angles of at least approximately 75.degree. and F numbers on the order of 2 since these lens systems are used most frequently in practice. When these lens systems are to be used with single-lens reflex cameras which inevitably comprise mirrors, however, it is obliged to select the retrofocus type for the lens systems. Since the retrofocus type lens systems have compositions which are asymmetrical with regard to stops, correction of negative distortion in particular poses a problem. Further problems are posed by correction of curvature of field when the lens systems are to have larger field angles and by correction of sagittal coma when the lens systems are to be brighter or have smaller F numbers.
As conventional wide-angle lens systems which are proposed for solving these problems, there are known the lens systems disclosed by Japanese Patents Kokai Publication No. Sho 47-41,626, Kokai Publication No. Sho 48-43,627, Kokai Publication No. Sho 50-87,643, Kokai Publication No. Sho 51-58332 and Kokai Publication No. Sho 61-10,045. Each of these lens systems consists of a front unit comprising a lens unit which consists of a plurality of lens components including at least one positive lens component disposed on the object side and has a negative refractive power as a whole, a stop, and a rear unit which comprises at least one negative lens component and two positive lens components disposed on the image side of the stop. It cannot be said that sagittal coma is corrected sufficiently in these lens systems and they have large sizes.
Further, known as other conventional examples are the lens systems disclosed by Japanese Patents Kokai Publication No. Sho 61-144,616, Kokoku Publication No. Sho 49-20,215, Kokoku Publication No. Sho 55-10,049, Kokoku Publication No. Sho 58-22,724 and Kokoku Publication No. Sho 60-40,009. Each of lens systems consists of a front unit comprising a lens unit which is composed of a plurality of lens components including at least one positive lens component and has a negative refractive power as a whole, a stop, and a rear unit which comprises at least one negative lens component and two positive lens components disposed on the image side of the stop; the rear unit using at least one aspherical surface. However, it cannot be said that negative distortion, curvature of field and sagittal coma are sufficiently corrected at the same time in these lens systems.
Furthermore, aberrations are varied remarkably by focusing the above-mentioned conventional examples onto objects located at short distances.
In addition, the lens system disclosed by Japanese Patent Kokoku Publication No. Sho 54-25,810 is known as a conventional example which adopts the floating moving mode for reducing the variations of aberrations to be caused by focusing the lens system onto an object located at a short distance. This conventional example is capable of reducing the variations of aberrations relatively favorably which are to be caused by focusing the lens system, but allows curvature of sagittal image surface and remarkable production of sagittal coma in particular. Therefore, it cannot be said that aberrations are corrected sufficiently in this lens system.